1. Field of the Invention
This invention relates to a local oscillator for use in communication systems that transmit and/or receive signals on multiple frequency bands and, in particular, it relates to such a local oscillator that uses a phase-locked loop (PLL hereinafter) circuit.
2. Discussion of the Prior Art
An illustrative, conventional local oscillator is shown in FIG. 1, the receiving side of an SSB transceiver being employed as an example. In FIG. 1, 1 is an antenna, 2 is a high frequency amplifier circuit, 3 is a mixing circuit, 4 is an intermediate frequency filter, 5 is an intermediate frequency amplifier circuit, 6 is a demodulation circuit, 7 is a local oscillator and 8 is a receiving frequency counter unit. These elements comprise the receiving unit of an SSB transceiver that receives signals on multiple frequency bands.
Local oscillator 7 consists of a PLL circuit which has two mixing circuits in a loop comprising a plurality of voltage control oscillators (VCO hereinafter) 9 equal to the number of the receiving frequency bands, two mixing circuits 10 and 11, a low-pass filter 12, a band-pass filter 13, a phase comparator 14, a loop filter 15 and a plurality of heterodyne crystal oscillators 16 equal to the number of VCOs 9.
Mixing circuit 10 mixes the output signal of the VCO 9 selected from the plurality of VCOs 9 corresponding to the receiving frequency band with the output signal of the heterodyne crystal oscillator 16 selected from the plurality of heterodyne crystal oscillators 16 corresponding to the receiving frequency to produce a signal having a frequency of a certain fixed range. The mixing circuit 11 mixes the output signal of mixing circuit 10 after it is passed by low-pass filter 12 with the output signal of a carrier frequency oscillator 17. Phase comparator 14 compares the phase of the output of mixing circuit 11 after it is passed by band-pass filter 13 with that of the output signal from a variable frequency oscillator (VFO hereinafter) 18, the output frequency of which is the reference frequency. The output from comparator 14 controls VCO 9 to produce the output of local oscillator 7.
Counter unit 8 comprises a mixing circuit 19 that mixes the output of VCO 9 with the output of carrier frequency oscillator 17, a counter 21 that receives the output of mixing circuit 19 and the output of a reference frequency oscillator 20, and an indicator 22 that displays the output of counter 21. Thus, unit 8 displays the receiving frequency.
However, in the case of a conventional local oscillator such as described above, there is a shortcoming in that the number of the heterodyne crystal oscillators has to equal the number of the receiving frequency bands.
Another shortcoming is that an error in the output frequency of a heterodyne crystal oscillator appears in the output causing the output frequency of the local oscillator to shift when the receiving frequency band is switched.
When the respective output frequencies of carrier frequency oscillator 17, VCO 9, heterodyne crystal oscillator 16, and VFO 18 are denoted as f.sub.CAR, f.sub.VCO, f.sub.HET, and f.sub.VFO, and the receiving frequency as f.sub.SIG, EQU f.sub.VCO =f.sub.HET +f.sub.CAR -f.sub.VFO, EQU f.sub.SIG =f.sub.VCO -f.sub.CAR, and EQU f.sub.SIG =f.sub.HET -f.sub.VFO
results. The receiving frequency can be indicated as the difference between the output frequency of heterodyne crystal oscillator 16 and the output frequency of VFO 18. However, in order to calibrate individual signals of the multiple crystal oscillators 16 into accurate oscillation frequencies, an enormous amount of labor is required resulting in high cost. Therefore, avoiding this method, the receiving frequency is indicated as the difference between the output frequency of VCO 9 and the output frequency of carrier frequency oscillator 17. Consequently, another shortcoming involves the complex construction of the counter unit.
In addition, because of the large number of the heterodyne crystal oscillators and the complex construction of the counter unit, another shortcoming is that the overall size increases making the product expensive.